I survived, but now some questions remain

[nellie]

hey...sheesh...stop complaining!! Those were some of the better photos!!

[bruce]

agreed' date=' not enough info to effectively provide an answer.

 

I assumed:

 

1) Constant speed of the truck.

2) torque  on the wheel was constant. However the torque at instant of release would provide the final speed of the wheel at the instant of departure so the torwue effect is Zero for the purpose of this discussion.

 

3) Friction and wind resistance are opposing forces.

4) the wheel did not gain mass by picking up crap on the road.

 

 

Under deceleration, the wheel  will also decelerate and it wil feed energy into the truck as well as the air and brake fluid and material.

At the instant of separation the deceleration my reduce but the nett effecgt is still deceleration, only at a different magnitude.

 

There is no positive acceleration of the wheel. It is an optical illusion.

[/quote']

 

Last sentence violates first assumption.

[TheLegend]

 

hey...sheesh...stop complaining!! Those were some of the better photos!!

 

you expect me to be thankful now?

 

tss

 

better check the web for your videoclips!

 

[Johan Bornman]

The problem is actually not well enough defined for any answer to be deemed correct.

 

Does the vehicle continue at the same speed' date=' or does it slow?  This changes the situation significantly, and from JB's perspective he would not have been able to tell whether the wheel accelerated or the vehicle decelerated, as both would appear the same from his point of view.

 

If you make the following assumptions:

- Vehicle continues at same speed.

- Wheel accelerates i.e. gains speed when it detaches.

 

Then it is clear that the balance of forces changes resulting in a nett forward force causing acceleration.  Assuming that rolling resistance decreases, this does not account for a nett positive force, it only accounts to a decrease in negative force (positive force being in the direction of motion, negative opposing motion).  Something must still be pushing the wheel forward.  Since the wheel is detached, the only possible source of this forward force is inertia, which we know the wheel has due to it's forward motion, and it's rotation.

 

As for the rotational energy of the wheel, the truck puts energy into the wheel when the wheel get's accelerated, and the wheel puts energy back into the truck when it decelerates.

 

[/quote']

 

Bruce, I really want to believe that the wheel accellerates. If you analyse the report by people who have a) seen this (but like you say from my perspective where realities are blurred) and b) who have experience this, then you start building a picture of acceleration.

 

None of these witnesses report a sensation of drag when the wheel departs, except for the trailer incident mentioned here. But that's easy to explain. The little bit of drag he felt was as the wheel jammed against the mudguard of the trailer. However, as soon as the wheel was free, the sensation was gone. The others all report they were totally unaware that something was amiss untill the wheel came past. My truck driver I chaced down in Robertson only saw a loss of tyre pressure but he didn't slow down because he didn't feel anything amiss. He was convinced it was a false reading since he felt  none of the side-effects of a flat tyre.

 

I know this is anecdotal and therefore dubious but...

 

Secondly, lets for a moment forget whether the wheel accellerates or not.

 

Lets look at the forces on the wheel before and after release.

 

Before release, the wheel has X momentum and effectively weighs 40 tons divided by 16 wheels = 2,5 tons.

 

After release, the wheel still has X momentum but only weighs 200kgs.

 

Lets examine what happens to that momentum after release.

 

Assumptions: Flat road, zero air drag before, some air drag afterwards.

 

 

 

 

[GoLefty!!]

agreed' date=' not enough info to effectively provide an answer.

 

I assumed:

 

1) Constant speed of the truck.

2) torque  on the wheel was constant. However the torque at instant of release would provide the final speed of the wheel at the instant of departure so the torwue effect is Zero for the purpose of this discussion.

 

3) Friction and wind resistance are opposing forces.

4) the wheel did not gain mass by picking up crap on the road.

 

 

Under deceleration, the wheel  will also decelerate and it wil feed energy into the truck as well as the air and brake fluid and material.

At the instant of separation the deceleration my reduce but the nett effecgt is still deceleration, only at a different magnitude.

 

There is no positive acceleration of the wheel. It is an optical illusion.

[/quote']

 

Last sentence violates first assumption.

 

 

no it does not because we are concerned with linear acceleration not rotational acceleration which is almost impossible to observe with the wheel heading straight for you...

[RockCoach]

Interesting discussion...

 

From first hand experience I know that a loose wheel does accelerate after detatching from a vehicle. I remember driving along the highway at about 120km/h (down towards Edenvale Hyperama) having just overtaken a bakkie going very slightly slower. I didn't see the wheel come off but I did observe it slowly overtake me. I don't think it was accelerating anymore but seemed to be travelling at a constant velocity.

 

I would go with the theory of slightly increased diameter. Perhaps the effect wouldn't be as marked in a low profile sportscar tyre as opposed to a heavy truck tyre with much higher profile with possibly more sidewall deformation when the load was removed.

 

While we are here, two other related anecdotes.

 

1. Talking about fearing for your life ... Five months ago, Sitting in KFC (Edenvale again) waiting for our order. The next thing I detect movement outside out of the corner of my eye. Something big and dark was moving straight  towards me at speed. It hit the window and the cracks seemed to spread in slow motion. But it couldn't have been that slow because the taxi wheel then bounced a few times harmlessly on the pavement before rolling in a circle to a stop. Thanks to safety glass I'm still here.

 

2. My grandparents used to live in 6th street Parkmore (Sandton). Early one morning they awoke to a huge crashing sound. A wheel had come off a taxi on 11th ave and crashed through the roof of one of their neighbours. Those that know the area will realise that this is quite a steep hill but still the distance covered and the obstacles (houses, walls, gardens etc) that this wheel bounced over is quite incredible.

 

[bruce]

agreed' date=' not enough info to effectively provide an answer.

 

I assumed:

 

1) Constant speed of the truck.

2) torque  on the wheel was constant. However the torque at instant of release would provide the final speed of the wheel at the instant of departure so the torwue effect is Zero for the purpose of this discussion.

 

3) Friction and wind resistance are opposing forces.

4) the wheel did not gain mass by picking up crap on the road.

 

 

Under deceleration, the wheel  will also decelerate and it wil feed energy into the truck as well as the air and brake fluid and material.

At the instant of separation the deceleration my reduce but the nett effecgt is still deceleration, only at a different magnitude.

 

There is no positive acceleration of the wheel. It is an optical illusion.

[/quote']

 

Last sentence violates first assumption.

 

 

no it does not because we are concerned with linear acceleration not rotational acceleration which is almost impossible to observe with the wheel heading straight for you...

 

If the wheel is in front of the vehicle, which is moving at constant velocity, then there must have been linear acceleration, as well as rotational acceleration because with no slippage or wheelspin the two are directly related.

[bruce]

The problem is actually not well enough defined for any answer to be deemed correct.

 

Does the vehicle continue at the same speed' date=' or does it slow?  This changes the situation significantly, and from JB's perspective he would not have been able to tell whether the wheel accelerated or the vehicle decelerated, as both would appear the same from his point of view.

 

If you make the following assumptions:

- Vehicle continues at same speed.

- Wheel accelerates i.e. gains speed when it detaches.

 

Then it is clear that the balance of forces changes resulting in a nett forward force causing acceleration.  Assuming that rolling resistance decreases, this does not account for a nett positive force, it only accounts to a decrease in negative force (positive force being in the direction of motion, negative opposing motion).  Something must still be pushing the wheel forward.  Since the wheel is detached, the only possible source of this forward force is inertia, which we know the wheel has due to it's forward motion, and it's rotation.

 

As for the rotational energy of the wheel, the truck puts energy into the wheel when the wheel get's accelerated, and the wheel puts energy back into the truck when it decelerates.

 

[/quote']

 

Bruce, I really want to believe that the wheel accellerates. If you analyse the report by people who have a) seen this (but like you say from my perspective where realities are blurred) and b) who have experience this, then you start building a picture of acceleration.

 

None of these witnesses report a sensation of drag when the wheel departs, except for the trailer incident mentioned here. But that's easy to explain. The little bit of drag he felt was as the wheel jammed against the mudguard of the trailer. However, as soon as the wheel was free, the sensation was gone. The others all report they were totally unaware that something was amiss untill the wheel came past. My truck driver I chaced down in Robertson only saw a loss of tyre pressure but he didn't slow down because he didn't feel anything amiss. He was convinced it was a false reading since he felt  none of the side-effects of a flat tyre.

 

I know this is anecdotal and therefore dubious but...

 

Secondly, lets for a moment forget whether the wheel accellerates or not.

 

Lets look at the forces on the wheel before and after release.

 

Before release, the wheel has X momentum and effectively weighs 40 tons divided by 16 wheels = 2,5 tons.

 

After release, the wheel still has X momentum but only weighs 200kgs.

 

Lets examine what happens to that momentum after release.

 

Assumptions: Flat road, zero air drag before, some air drag afterwards.

 

 

 

 

 

JB, weight is acting 90degrees to the direction of motion, and therefore cannot contribute to the nett acceleration in the horizontal plane.

[GoLefty!!]

agreed' date=' not enough info to effectively provide an answer.

 

I assumed:

 

1) Constant speed of the truck.

2) torque  on the wheel was constant. However the torque at instant of release would provide the final speed of the wheel at the instant of departure so the torwue effect is Zero for the purpose of this discussion.

 

3) Friction and wind resistance are opposing forces.

4) the wheel did not gain mass by picking up crap on the road.

 

 

Under deceleration, the wheel  will also decelerate and it wil feed energy into the truck as well as the air and brake fluid and material.

At the instant of separation the deceleration my reduce but the nett effecgt is still deceleration, only at a different magnitude.

 

There is no positive acceleration of the wheel. It is an optical illusion.

[/quote']

 

Last sentence violates first assumption.

 

 

no it does not because we are concerned with linear acceleration not rotational acceleration which is almost impossible to observe with the wheel heading straight for you...

 

If the wheel is in front of the vehicle, which is moving at constant velocity, then there must have been linear acceleration, as well as rotational acceleration because with no slippage or wheelspin the two are directly related.

 

Don't change the scenario. Never did I say the wheel is in front of the vehicle.

We don't know if the vehicle slowd down or not. YOu are adding information to suit your arguement. Read the scenario i poted again.

If the vehicle is at constant velocity then the wheel will not accelerate away from it and run ahead. It cannot because the retarding froces have not bee removed nor changed direction, they have only reduced in magnitude but are still retarding forces.

 

anOther question to ask;

was the road flat or was it inclined?

 

[bruce]

agreed' date=' not enough info to effectively provide an answer.

 

I assumed:

 

1) Constant speed of the truck.

2) torque  on the wheel was constant. However the torque at instant of release would provide the final speed of the wheel at the instant of departure so the torwue effect is Zero for the purpose of this discussion.

 

3) Friction and wind resistance are opposing forces.

4) the wheel did not gain mass by picking up crap on the road.

 

 

Under deceleration, the wheel  will also decelerate and it wil feed energy into the truck as well as the air and brake fluid and material.

At the instant of separation the deceleration my reduce but the nett effecgt is still deceleration, only at a different magnitude.

 

There is no positive acceleration of the wheel. It is an optical illusion.

[/quote']

 

Last sentence violates first assumption.

 

 

no it does not because we are concerned with linear acceleration not rotational acceleration which is almost impossible to observe with the wheel heading straight for you...

 

If the wheel is in front of the vehicle, which is moving at constant velocity, then there must have been linear acceleration, as well as rotational acceleration because with no slippage or wheelspin the two are directly related.

 

Don't change the scenario. Never did I say the wheel is in front of the vehicle.

We don't know if the vehicle slowd down or not. YOu are adding information to suit your arguement. Read the scenario i poted again.

If the vehicle is at constant velocity then the wheel will not accelerate away from it and run ahead. It cannot because the retarding froces have not bee removed nor changed direction, they have only reduced in magnitude but are still retarding forces.

 

anOther question to ask;

was the road flat or was it inclined?

 

 

So you are saying that it is an optical illusion that the wheel is in front of the vehicle.  Given the assumption that the vehicle has remained at constant speed.  Which makes the entire discussion a moot point.

[GoLefty!!]

I think I made the point that the wheel being infrontof the vehicle is an optical illusion.

 

If hte vehicles heading directly toward the observer they are on collision course.

then we can assume 2 things:

 

1) the vehicle is slowing down and decelerating not under constant velocity, hence the wheel appears to accelerate away from it. relativity and a bit of Dopler effect.

 

2) the oncoming vehicle is accelerating toward the wheel. In this case the driver says the vehicle is stationary.

 

If you are a fielder at a game of cricket and the batsman whacks the ball at you on the boundary. If you have to run at an angle toward the ball, depending ontheangle  the ball will appear to be accelerating toward you or decelerating toward you.

In both cases it is decelerating because the acceleration it needed was input into it during contact with the bat. the appearance of acceleration is due to the point of view of the observer.

[bruce]

I think I made the point that the wheel being infrontof the vehicle is an optical illusion.

 

 

I just wanted to confirm my understanding of your viewpoint.

[Johan Bornman]

[quote name=bruce}

 

 

 

JB' date=' weight is acting 90degrees to the direction of motion, and therefore cannot contribute to the nett acceleration in the horizontal plane.

[/quote]

 

OK then, look at the converse. Weight acting at 90 degrees off-plane to forward motions has a direct bearing on the deformation of the tyre and a direct bearing on the rolling resistance. I.e. it is a force that acts in the rearward direction.

 

 

[bruce]

 

 

 

JB' date=' weight is acting 90degrees to the direction of motion, and therefore cannot contribute to the nett acceleration in the horizontal plane.

[/quote']

 

OK then, look at the converse. Weight acting at 90 degrees off-plane to forward motions has a direct bearing on the deformation of the tyre and a direct bearing on the rolling resistance. I.e. it is a force that acts in the rearward direction.

 

 

 

Sure, but where is the force acting in the forward direction coming from, in order for there to be a nett positive force?  GoLefty!!  and I are in agreement on this point.

 

Furthermore, rolling resistance is not keeping the wheel going the same speed as the truck when it is attached to the truck, the fact that it is attached to the truck is what is keeping it going the same speed.

 

 

bruce2008-11-17 08:12:11

[greatwhite]

OK Guys... Bite size pieces.....

 

1. Can the wheel accelarate if no additional energy is added? (Y/N)

[GoLefty!!]

I think I made the point that the wheel being infrontof the vehicle is an optical illusion.

 

 

I just wanted to confirm my understanding of your viewpoint.

 

Ah we're on the same page then